As illustrated in FIGS. 12 and 13, a typical front floor panel assembly 21 for forming a front floor part of an automotive body typically consists of a front floor section 22, a dashboard section 23, and right and left side sill sections 24 and 25. A center tunnel section 26 projects upward from a laterally middle part of the front floor section 22 and extends over the entire length of the front floor section 22. The center tunnel section 26 provides a high rigidity and a high mechanical strength against bending deformation of the front floor panel assembly, and is also used for accommodating an exhaust pipe therein. In the case of a front engine rear drive vehicle, it can be used for accommodating a propeller shaft.
According to the conventional structure, the front floor section 22 and the side sill sections 24 and 25 are stamped out of sheet metal stock of different thicknesses, and are joined together along their edges by spot welding.
The dashboard section 23 must be adapted either to right hand side cars or to left hand side cars, and two different kinds of dashboard sections 23 must be prepared when both right hand side cars and left hand side cars are required to be assembled. Also, because the dashboard section 23 is relatively large and heavy, the assembling of the dashboard section 23 to the floor panel section 22 presented some problem in the production efficiency.
Additionally, slight gaps are inevitable in the spot welded parts of the front floor section 22 and the side sill sections 24 and 25, and a sealant must be carefully applied to these parts to exclude any intrusion of dust and moisture into the inner hollow sections of the front floor panel assembly. This means a substantial increase in the productions cost.
The center tunnel section and the floor panel sections are stamped from a same sheet of metal, and have a same thickness. Therefore, to ensure sufficient rigidity and mechanical strength, a stiffener 27 made of sheet metal having a relatively large thickness is spot welded to the center tunnel section 26 or the front floor sections 22. The stiffener 27 typically consists of front floor frame members 28 and 29 spot welded along either side of the center tunnel section 26. This caused a substantial increase in the weight of the front floor panel.
As illustrated in FIGS. 14 and 15, a typical rear floor panel assembly 41 for forming a rear floor part of an automotive body comprises a rear floor section 42, and right and left rear wheel house sections 43 and 44. According to the conventional structure, the rear floor section 42 and the rear wheel house sections 43 and 44 are stamped from sheet metal stock of different thicknesses, and a flange 42a of the rear floor section 42 and a flange 43a of the fight rear wheel house section 43 are spot welded, and flanges 42a and 44a of the rear wheel section 42 and the left rear wheel house section 44 are similarly spot welded. Alternatively, the rear floor panel section 42 and the rear wheel house sections 43 and 44 may be integrally stamped from a single sheet of sheet metal.
When the rear floor panel 41 consists of three pieces of sheet metal which are spot welded together, the assembling work involved was highly difficult to carry out due to the complexity of the shapes of the parts to be welded, and the tortuous welding paths, and it was highly difficult to maintain the welding quality at a satisfactory level at all times. It was therefore often necessary to use bonding agents to supplement the function of the welding. As a result, the assembling work became highly complex, and the production cost accordingly was substantial.
When the rear floor panel is made of a single piece of sheet metal, the production process would be simplified. But, because the entire panel is of uniform thickness, a special measure must be taken to shield road noises from the rear tires. Typically, sound insulating/damping sheets are attached to the wheel houses. This adds to the weight and cost of the rear floor panel. It obviously would not be acceptable to increase the thickness of the entire rear floor panel.
Furthermore, stamping the entire rear wheel panel from a single piece of sheet metal involves a certain difficulty depending on the suitability of the sheet metal stock to draw work, and the depth of the wheel houses. For instance, a recess for receiving a spare tire cannot be made as deep as desired, and the recess may be made wider than actually necessary. As a result, the rear frame members which are attached to the lower surface of the rear floor panel and extend longitudinally on either side of the recess for a spare tire must be offset laterally to provide flange surfaces for spot welding without interfering with the spare tire recess. This will create some difficulty in passing the rear frame members longitudinally through the rear part of the vehicle, and may necessitate the rear frame members to be curved along either side of the spare tire recess. The resulting reduction in the mechanical strength of the rear frame members must be compensated for by using appropriate reinforcement members, and this will add to the production cost and the weight of the vehicle body.